This invention relates to a multi-layer film structure comprising a polyolefin core layer, an ethylene-vinyl alcohol copolymer (EVOH) skin, and an acrylic coating.
Acrylic copolymers have been used as heat sealable coatings for oriented thermoplastic films such as polypropylene film. When thermoplastic film substrates are coated with these heat sealable coatings, it is particularly important to insure that the adhesion of the coating layer to the film substrate is adequate. In many packaging operations it is necessary for the coated film to be heat sealed to itself or to other films to form tightly closed packaging. If the coating adhesion to the base film is inadequate, the packages may prematurely open when subjected to stress.
In general, acrylic coatings do not adhere well to certain polyolefin film surfaces, e.g., polypropylene, even when the latter have been subjected to well known pretreatment operations such as treatment by corona discharge, flame, or oxidizing chemicals. Accordingly, it has often been found necessary to apply a thin intermediate primer layer to the surface of the polyolefin base film before applying heat sealable topcoats. Moreover, the acrylic coatings themselves are not especially effective as oxygen and aroma barriers.
Ethylene vinyl alcohol copolymer (EVOH) films are excellent barriers to oxygen, carbon dioxide and nitrogen. They are also effective barriers against odors and the loss of flavor. EVOH resins, however, are moisture sensitive and their barrier properties are reduced in the presence of high humidity.
Biaxially oriented coextruded multilayer films in which an EVOH layer is sandwiched between two polyolefin layers, e.g., polypropylene or high density polyethylene (HDPE) layers, are described in U.S. Pat. No. 4,561,920 which is incorporated herein by reference.
U.S. Pat. No. 4,880,706 to Mazuera et al. discloses multilayer films which are extruded to provide a core layer of ethylene-vinyl alcohol copolymer (EVOH) which is encapsulated on its top and bottom surfaces by a polyolefin layer, e.g. polypropylene. The extrudate is cooled and subjected to machine direction orientation to a degree greater than four times, and to transverse direction (TD) orientation to a degree greater than seven times.
It would be advantageous to provide an acrylic coated polyolefin film which enjoys not only enhanced sealing characteristics but enhanced oxygen and aroma barrier as well. Moreover, it is desirable to provide a method for reducing the minimum seal temperature of a heat sealable film in order to permit heat sealing at lower temperatures which is advantageous in that it provides a wider seal range and lower heat settings on the sealing bar which is especially advantageous when running products that melt, such as chocolate.